The present invention relates to alpha-numeric cathode ray tube (CRT) displays. More particularly, it relates to such a display where a number of different formats can be presented on the screen.
With the introduction of programmable CRT controller modules (CRTC's) flexibilty has been given to CRT display screen formatting. The number of characters per row, the rows per screen etc. can all be changed at any time by the display operator. When there is such a change in the CRTC will generate the required addresses for the display buffer memory and the synchronizing pulses for the CRT analog drive circuits to attain the new display format selected by the operator. Presently used analog CRT display circuits are not really suitable for variable display formating. Those display circuits were adapted from similar circuits used in television monitors where their use was limited to a fixed display format and as a result changes in the format of the display cause undesirable changes in what appears on the screen. For instance, when the number of characters on a line is reduced, letters will become broad while increasing the number of lines on the screen will result in narrower letters. Furthermore, variations in frequency of horizontal synchronizing pulses affects the width of the display frame so that part of the data will be off screen without operator adjustment of the display. Vertical synchronizing pulses are also frequency sensitive and can also require operator display adjustment.